You don't understand your ability very well at all. If you died while using it then you must have clicked it as absurdly low HP and if you ever lose 50% health when you activate it then you need to L2P. Using it at full health is LOL.
And FYI there is a LOT of difference between UR and a stun ability. How can you even compare the two? Really guy? You really don't have a grasp of your class if you would trade UR for a 4 sec hard stun.
I believe his point is that UR functions similarly to a hard stun, but not as effective. He is both correct and incorrect in this statement. He is correct in that you could stun someone when you have almost no health and beat on them for 4 seconds to win the fight just as easily as popping UR. The negative to UR is that it can only be done at the end of a fight without sacrificing massive amounts of health, so it is indeed less effective than a stun. The downside to a stun, however, is that it only affects one opponent whereas UR mitigates damage by 99% on anyone who happens to attack. In a 1v1 situation, the stun is better, but in a group setting UR is still a superior ability.